Method and device for substrate etching with very high power inductively coupled plasma

ABSTRACT

According to the invention, etching is performed in a reaction chamber ( 1 ) by subjecting a substrate ( 16 ) biased by a bias generator ( 15 ) to a plasma generated by a plasma source ( 4 ) contained in a leakproof wall ( 5 ) of dielectric material surrounded by an inductive coupled antenna ( 6 ) powered by a radiofrequency generator ( 7 ). Control means ( 13 ) control solenoid valves ( 12   a,    12   b,    12   c ) and the radiofrequency generator ( 7 ) so as to produce a prior step of establishing the plasma excitation power progressively, during which step an inert gas such as argon or nitrogen is injected into the reaction chamber ( 1 ), and the power delivered by the radiofrequency generator ( 7 ) is raised progressively until it reaches a nominal power. This avoids applying thermal shock to the leakproof wall ( 5 ) of dielectric material that might otherwise destroy the wall, thus making it possible to plasma excitation powers that are greater than 3000 W.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to methods and apparatuses for etchingsubstrates, for example in the reactors used for implementingmicromachining or etching methods on a silicon substrate.

When a silicon substrate is etched in a plasma reactor, the sequencesare as follows:

-   -   after the substrate has been inserted and positioned on a sample        carrier contained in a reaction chamber, the etching gas(es),        such as a fluorine-containing gas like SF₆, is/are introduced at        a pre-established rate;    -   an appropriate pressure is established in the reaction chamber        by means of a pump unit and a pressure servo-control system, and        said pressure is maintained; and    -   once the pressure has stabilized, the gas in the reaction        chamber is excited by an excitation electromagnetic wave for        generating a plasma; simultaneously, the substrate on the sample        carrier is biased to accelerate the ions which then bombard the        surface of the substrate that is being etched.

In micromachining applications, it is desired to etch the silicon asquickly as possible. Amongst the parameters that are accessible forcontrolling etching speed, the parameters having the most influence arethe following:

-   -   the partial pressure of halogen-containing gas ions such as SF₆;    -   the power of the electromagnetic wave for exciting the gas.

The power of the excitation electromagnetic wave serves to ionize anddissociate the halogen-containing gas molecules such as SF₆ so as togenerate fluorine atoms. On reaching the surface of the siliconsubstrate, these fluorine atoms react therewith to form gaseousmolecules in application of the reaction:Si(s)+4F(g)→SiF₄(g)

Etching thus consists in taking atoms of silicon from the substratewhich are transformed by the reaction into a gas SiF₄, which gas is thenremoved from the reaction chamber by the pump means.

It will be understood that the speed at which the silicon is etched isdirectly proportional to the pressure of atomic fluorine, and thus tothe dissociation ratio of the halogen-containing gas molecules such asSF₆.

Amongst the various types of plasma source, the following sources areknown: reactive ion etching (RIE) sources, electron cyclotron resonance(ECR) sources, and inductively-coupled plasma (ICP) sources. The sourcesthat present the greatest dissociation ratio under high pressureconditions are ICP sources, thereby making it possible to have both ahigh dissociation ratio and a high partial pressure of atoms from thehalogen-containing gas such as SF₆ in the reaction chamber.

It is therefore natural to use an ICP source in order to increase thespeed at which the silicon is etched.

ICP type plasma sources are all constituted by two main elements:

-   -   a leakproof wall of dielectric material which closes the        reaction chamber in leakproof manner; and    -   an antenna made of electrically-conductive material such as        copper, surrounding or surmounting the leakproof wall made of        dielectric material; the antenna is connected at one of its ends        to the electrical ground of the equipment, and at its other end        to a radiofrequency (RF) power generator via an automatic        impedance matcher.

The leakproof wall of dielectric material is connected to the remainderof the wall of the reaction chamber, which is generally made of metal,via gaskets that are generally made of polymer type materials. Suchmaterials have maximum working temperatures in continuous utilizationthat do not exceed 150° C. As a result, the zone of the reaction chamberwall that is close to the gaskets is cooled.

During a process of etching a substrate such as silicon, the quality ofthe etching depends on all of the etching parameters being adjusted tospecific values at all times, and in particular the pressure of theetching gas, and also the power of the excitation electromagnetic wavetransmitted to the gas in order to generate the plasma. The etchingsequences are run one after another over a time interval of the order ofa few milliseconds (ms).

Consequently, at the plasma source, there is a situation where it isnecessary to produce quasi-instantaneous inductive coupling of thenominal RF power to the plasma through the leakproof wall of dielectricmaterial.

Until now, quasi-instantaneous inductive coupling of the excitationelectromagnetic wave has been possible up to powers of about 2000 watts(W), by using leakproof walls made of dielectric material that withstandhigh temperatures. Alumina Al₂O₃ has been used with success.

Nevertheless, such a material does not enable quasi-instantaneousconductive coupling to be achieved with an excitation electromagneticwave at a power greater than a maximum of about 3000 W, since otherwisethe plasma source is destroyed quasi-instantaneously: the leakproof wallof dielectric material cracks, thereby returning the inside of theetching reactor to atmospheric pressure, and possibly leading to theassembly imploding, and thus being destroyed.

Thus, at present, there is no solution that enables very high powers tobe coupled in quasi-instantaneous manner through a dielectric materialsuch as alumina.

SUMMARY OF THE INVENTION

An object of the present invention is to avoid the drawbacks of priorart structures and methods for etching a substrate by aninductively-coupled plasma, by making it possible to couple RF powers upto 5000 W through a dielectric material such as alumina.

Simultaneously, the invention seeks to conserve good quality for theetching, avoiding the use of etching steps in which the parameters arenot maintained precisely at their nominal values.

The idea on which the invention is based is to reduce the thermal shockto the dielectric material constituting the plasma source, by couplingthe power of the excitation electromagnetic wave gradually. A power riseramp is thus used with the slope of the ramp being sufficiently gentleto avoid creating a destructive thermal shock.

However, since etching quality and performance depend on the values ofmachine parameters such as RF power, it is not possible to envisagetriggering the etching plasma and then causing power to riseprogressively while the substrate is in position on the biased samplecarrier: that would lead throughout the power rise stage to plasmaconditions that are extremely variable and harmful to obtaining goodetching performance.

According to the invention, power is raised progressively, but in thepresence of an inert gas such as nitrogen or argon, so that there is noreaction between said gas and the silicon sample.

The sole function of the inert gas is to enable a plasma to be generatedwhich, under the effect of the progressive rise in power, serves to heatthe dielectric material progressively, thereby bringing it to itsworking temperature corresponding to the maximum power that is usedthroughout the step of etching by means of a plasma of reagent gas.

After this step of raising the temperature of the dielectric material bymeans of a plasma of inert gas, it is possible to stop injecting theinert gas and to switch over instantly to a halogen-containing reagentgas such as SF₆ in order to perform etching proper.

To achieve these objects, and others, the invention provides a method ofetching a substrate by an inductively-coupled plasma, in which thesubstrate is placed in a reaction chamber, an atmosphere of anappropriate gas is established in the reaction chamber at a suitableoperating pressure, the substrate is biased, and the gas in the reactionchamber is excited by a radiofrequency excitation electromagnetic wavepassing through a leakproof wall of dielectric material in order togenerate a plasma; according to the invention, the method includes aprior step of establishing the power of the plasma excitationelectromagnetic wave progressively, during which step a gas that isinert for the substrate is injected into the reaction chamber and thepower of the plasma excitation electromagnetic wave is raisedprogressively until the appropriate nominal power is reached, therebyforming an inert gas plasma which progressively heats up the leakproofwall of dielectric material, after which active gas is injected into thereaction chamber in order to replace the inert gas and undertake activesteps of etching by means of the plasma of active gas.

Preferably, the progressive increase in the plasma excitation power isprogrammed so as to ensure that the thermal shock applied to theleakproof wall of dielectric material by the inert gas plasma remainsbelow a wall-destroying threshold.

When possible, the prior step of progressively establishing the plasmaexcitation power is undertaken solely at the beginning of reactionchamber operation after a period of inactivity, and is followed byalternating active etching steps during which the temperature of theleakproof wall of dielectric material remains in a range of values thatis sufficiently narrow to avoid any destructive thermal shock beingapplied to the leakproof wall of dielectric material.

The active etching steps may comprise a succession of etching stepsusing a fluorine-containing gas such as SF₆, and passivation steps usinga of etching passivation gas such as C_(x)F_(y).

The invention also provides apparatus for etching substrates by aninductively-coupled plasma implementing the method as defined above, theapparatus comprising a reaction chamber surrounded by a leakproof wall,the reaction chamber having substrate support means and being incommunication with an inductively-coupled plasma source having aleakproof wall of dielectric material and an inductive coupling antennapowered by a radiofrequency generator, the reaction chamber beingconnected via a vacuum line to pump means for establishing andmaintaining an appropriate vacuum inside the reaction chamber, thereaction chamber being connected via an inlet line to a process gassource; according to the invention:

-   -   the process gas source comprises an inert gas source, at least        one active gas source, and distribution means controlled by        control means to introduce the appropriate gas into the reaction        chamber;    -   the radiofrequency generator has means for adjusting its        radiofrequency power under the control of the control means; and    -   the control means include a control program with a prior        sequence of establishing power, during which:        -   a) the control means control the distribution means to            introduce an inert gas into the reaction chamber;        -   b) the control means cause the radiofrequency power control            means of the radiofrequency generator to produce            radiofrequency energy that increases progressively until            reaching the nominal power; and        -   c) thereafter the control means control the distribution            means to replace the neutral gas in the reaction chamber            with an active gas.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, characteristics, and advantages of the present inventionappear from the following description of particular embodiments, givenwith reference to the accompanying figures, in which:

FIG. 1 is a diagrammatic view showing the general structure of etchingapparatus in an embodiment of the present invention; and

FIG. 2 is a timing diagram showing the operation of the main members ofthe FIG. 1 apparatus, diagram a) showing variation in the plasmaexcitation power; diagram b) showing the feed of inert gas to thereaction chamber; diagram c) showing the feed of etching gas to thereaction chamber; diagram d) showing the feed of passivation gas to thereaction chamber; and diagram e) showing the bias applied to thesubstrate for etching.

DESCRIPTION OF PREFERRED IMPLEMENTATIONS

Reference is made initially to the apparatus shown in FIG. 1. There canbe seen a reaction chamber 1 surrounded by a leakproof wall 2. Thereaction chamber 1 contains substrate support means 3 suitable forreceiving and holding a substrate 16 for etching. The reaction chamber 1is in communication with an inductively-coupled plasma source 4constituted by a leakproof wall 5 of dielectric material associated withan inductive coupling antenna 6 powered by an RF generator 7 via animpedance matcher 7 a.

The reaction chamber 1 is connected by a vacuum line 8 to pump means 9for establishing and maintaining a suitable vacuum inside the reactionchamber 1. The reaction chamber 1 is connected by an inlet line 10 to asource of process gas 11.

In the embodiment shown, the leakproof wall 2 of the reaction chamberhas a peripheral portion 2 a which is connected to an inlet frontportion 2 b which is itself open in order to communicate with an inlettube constituting the plasma source 4.

This plasma source 4, in the embodiment shown, is constituted by aleakproof wall 5 of dielectric material and of tubular shape, and theinductive coupling antenna 6 is a coaxial turn of electricallyconductive material disposed around the tubular wall, and connectedfirstly to apparatus ground 6 a and secondly to the outlet of theimpedance matcher 7 a.

The inductive coupling antenna 6 is placed around the central portion ofthe tubular leakproof wall 5 of dielectric material, itself constitutedby alumina Al₂O₃.

To connect the tubular leakproof wall 5 of dielectric material with theinlet front portion 2 b of the reaction chamber 1, which portion 2 b isgenerally made of metal, a sealing gasket 2 c is provided. Cooling means2 d are also provided to enable the inlet front portion 2 b and thesealing gasket 2 c to be cooled.

The substrate 16 held on the substrate support means 3 is biased by abias generator 15 in conventional manner.

The process gas source 11 comprises an inert gas source 11 a, and atleast one source of active gas. For example, a first active gas source11 b is provided containing a fluorine-containing gas such as SF₆ foretching purposes, and a second active gas source 11 c is providedcontaining a passivation gas such as C₄F₈.

Distribution means serve to control the introduction of an appropriategas into the reaction chamber 1. The distribution means comprisessolenoid valves 12 a, 12 b, and 12 c each connected in series betweenthe outlet of a corresponding gas source 11 a, 11 b, or 11 c, and aninlet 14 to the plasma source 4.

The RF generator 7 has means for adjusting its RF power, under thecontrol of control means 13. Similarly, the distribution means 12 a, 12b, and 12 c are controllable by the control means 13.

Control means 13 are provided, e.g. a micro-controller with inlet/outletmembers, and associated with a controlling program, that is adapted tocontrol the distribution means having solenoid valves 12 a-12 c and theRF generator 7.

The control means 13 have a control program 13 a with a prior sequencefor running up to power during which:

a) the control means 13 cause the distribution means to open the inertgas valve 12 a so as to introduce an inert gas such as nitrogen N₂ orargon into the reaction chamber 1;

b) the control means 13 cause the RF power adjustment means of the RFgenerator 7 to produce RF energy which increases progressively until itreaches nominal power PN, so as to produce a plasma 24 in the plasmasource 4 in order progressively to raise the temperature of theleakproof wall 5 of dielectric material of the plasma source; and

c) once the leakproof wall 5 has been heated sufficiently, the controlmeans 13 cause the distribution means to close the inert gas valve 12 aand open a valve 12 b or 12 c for delivering active gas. In practice,the etching gas valve 12 b and the passivation gas valve 12 c are openedsequentially so as to introduce the active gases into the reactionchamber 1, and the control means 13 simultaneously control the means foradjusting the RF power of the RF generator 7 so as to produce the plasma24 that is appropriate for the etching steps and for the passivationsteps.

Reference is now made to FIG. 2 which shows the steps in an etchingmethod in an implementation of the invention.

After placing the substrate 16 (FIG. 1) in the reaction chamber 1, anatmosphere of inert gas such as nitrogen N₂ or argon is established inthe reaction chamber: at instant A, diagram b) indicates the presence ofnitrogen during a first step that continues unit instant B. During thisstep, the pump means 9 establish and maintain a suitable pressure insidethe reaction chamber 1, which pressure is selected to enable a plasma 24to be established properly. During this step, the substrate 16 is notbiased, as can be seen from diagram e) in FIG. 2: the bias voltage V isabsent throughout the step between instants A and B. During this samestep, the plasma excitation power is established progressively, as shownin diagram a) of FIG. 2, e.g. by increasing power in linear mannerbetween instants A and B until the nominal power PN is reached atinstant B.

At instant B, or after an additional delay determined to ensure that theleakproof wall 5 is sufficiently heated, the introduction of inert gassuch as nitrogen or argon is interrupted, as represented by way ofexample in diagram b) which shows the end of the presence of nitrogen asfrom instant B.

At the same instant B, or after the above-mentioned additional delay, ahalogen-containing etching gas such as SF₆ is introduced into thereaction chamber 1 and the presence of that gas is maintained during astep BC of duration that is appropriate as a function of the desiredetching process. During this step, the substrate is biased by a voltageV as shown in diagram e), with the bias voltage possibly beingestablished with a suitable delay relative to the presence of etchinggas SF₆ becoming established. Thereafter, at instant C, the etching gasSF₆ is replaced by a passivation gas such as C₄F₈, and diagram c) showsthe disappearance of SF₆ while diagram d) shows the appearance of C₄F₈and shows that it is maintained until instant D. During this step CD,the passivation gas causes polymer to be deposited on the surfaces ofthe substrate. Etching steps and passivation steps are subsequentlyalternated, as shown in the diagrams, with the substrate being biasedeach time to attract the plasma 24, and with the plasma excitation powerbeing maintained at a suitable value that may be close to the nominalvalue PN.

Thus, the prior step of progressively establishing the plasma excitationpower is undertaken only at the beginning of operation of the reactionchamber 1 after it has been inactive for some length of time, and it isthen followed by active steps of etching, e.g. alternating etching stepsand passivation steps, during which the temperature of the leakproofwall 5 of dielectric material remains in a range of values that issufficiently narrow to avoid any thermal shock that might destroy theleakproof wall 5 of dielectric material.

During the prior step of progressively establishing the plasmaexcitation power, between instants A and B, the power rise slope asshown in diagram a) is selected to be sufficiently shallow to avoid anyrisk of the leakproof wall 5 of dielectric material being destroyed bythe plasma of inert gas.

By using an inert gas, such as nitrogen N₂ or argon, it is ensured thatthe inert gas plasma 24 does not act on the substrate 16 that is to beetched, thereby concerning etching of good quality. Preferably duringthis step, the substrate 16 is also not biased, so as to avoid thesubstrate 16 being bombarded by the plasma.

By using the means of the invention, it is possible without destroyingthe plasma source 4 and its leakproof wall 5 of dielectric material, toestablish RF power greater than 3000 W, thereby enabling etching to beperformed at a higher speed. Satisfactory tests have been undertakenwith RF powers up to 5000 W, passing through a dielectric material suchas alumina.

The invention is not limited to the embodiments explicitly describedabove, but it includes various variants and generalizations that arewithin the competence of the person skilled in the art.

1. A method of etching a substrate (16) by an inductively-coupled plasma(24), in which the substrate (16) is placed in a reaction chamber (1),an atmosphere of an appropriate gas is established in the reactionchamber (1) at a suitable operating pressure, the substrate (16) isbiased, and the gas in the reaction chamber (1) is excited by aradiofrequency excitation electromagnetic wave passing through aleakproof wall (5) of dielectric material in order to generate a plasma(24), which method is characterized in that it includes a prior step ofestablishing the power of the plasma excitation electromagnetic waveprogressively, during which step a gas that is inert for the substrateis injected into the reaction chamber (1) and the power of the plasmaexcitation electromagnetic wave is raised progressively until theappropriate nominal power is reached, thereby forming an inert gasplasma (24) which progressively heats up the leakproof wall (5) ofdielectric material, after which active gas is injected into thereaction chamber (1) in order to replace the inert gas and undertakeactive steps of etching by means of the plasma (24) of active gas.
 2. Amethod according to claim 1, characterized in that the progressiveincrease in the plasma excitation power is programmed so as to ensurethat the thermal shock applied to the leakproof wall (5) of dielectricmaterial by the inert gas plasma (24) remains below a wall-destroyingthreshold.
 3. A method according got claim 1, characterized in that theprior step of progressively establishing the plasma excitation power isundertaken solely at the beginning of reaction chamber operation after aperiod of inactivity, and is followed by alternating active etchingsteps (BC; CD) during which the temperature of the leakproof wall (5) ofdielectric material remains in a range of values that is sufficientlynarrow to avoid any destructive thermal shock being applied to theleakproof wall (5) of dielectric material.
 4. A method according toclaim 1, characterized in that the active etching steps comprise asuccession of etching steps (BC) using a fluorine-containing gas such asSF₆, and passivation steps (CD) using a of etching passivation gas suchas C_(x)F_(y).
 5. Apparatus for etching substrates (16) by aninductively-coupled plasma, the apparatus implementing a methodaccording to any one of claims 1 to 4, and comprising a reaction chamber(1) surrounded by a leakproof wall (2), the reaction chamber (1) havingsubstrate support means (3) and being in communication with aninductively-coupled plasma source (4) having a leakproof wall (5) ofdielectric material and an inductive coupling antenna (6) powered by aradiofrequency generator (7), the reaction chamber (1) being connectedvia a vacuum line (8) to pump means (9) for establishing and maintainingan appropriate vacuum inside the reaction chamber (1), the reactionchamber (1) being connected via an inlet line (10) to a process gassource (11), the apparatus being characterized in that: the process gassource (11) comprises an inert gas source (11 a), at least one activegas source (11 b, 11 c), and distribution means (12 a, 12 b, 12 c)controlled by control means (13) to introduce the appropriate gas intothe reaction chamber (1); the radiofrequency generator (7) has means foradjusting its radiofrequency power under the control of the controlmeans (13); and the control means (13) include a control program (13 a)with a prior sequence of establishing power, during which: a) thecontrol means (13) control the distribution means (12 a, 12 b, 12 c) tointroduce an inert gas into the reaction chamber (1); b) the controlmeans (13) cause the radiofrequency power control means of theradiofrequency generator (7) to produce radiofrequency energy thatincreases progressively until reaching the nominal power (PN); and c)thereafter the control means (13) control the distribution means (12 a,12 b, 12 c) to replace the neutral gas in the reaction chamber (1) withan active gas.
 6. Apparatus according to claim 5, characterized in thatthe distribution means (12 a, 12 b, 12 c) comprise solenoid valves eachconnected in series between a respective corresponding gas source outlet(11 a, 11 b, 1 c) and an inlet (14) to the plasma source (4). 7.Apparatus according to claim 5, characterized in that it includes asource (11 a) of inert gas such as nitrogen (N₂) or argon, a source (11b) of an etching gas such as SF₆, and a source (11 c) of a passivationgas such as C₄F₈.
 8. Apparatus according to claim 5, characterized inthat the leakproof wall (5) of dielectric material of the plasma source(4) is made of alumina Al₂O₃.
 9. Apparatus according to claim 5,characterized in that the leakproof wall (5) of dielectric material ofthe plasma source (4) is of tubular shape, and the inductive couplingantenna (6) is a coaxial turn placed around the tubular wall. 10.Apparatus according to claim 5, characterized in that the leakproof wall(2) of the reaction chamber (1) has a peripheral portion (2 a) connectedto an inlet front portion (2 b) that is itself open to communicate withan inlet tube constituting the plasma source (4), the inlet frontportion (2 b) being connected to the leakproof wall (5) of dielectricmaterial by means of a sealing gasket (2 c), together with cooling means(2 d) for cooling the inlet front portion (2 b) and the sealing gasket(2 c).